1. Field of the Invention
The present invention generally relates to a motion vector detector and a motion vector detecting method, and more particularly to a motion vector detector and a motion vector detecting method capable of correcting a motion vector matching its original motion in the block matching technique.
This application claims the priority of the Japanese Patent Application No. 2002-101766 filed on Apr. 3, 2002, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
Heretofore, the TV broadcasting systems have adopted most widely the interlaced scanning or interlacing in which the electron beam is scanned across the screen once every other horizontal scan line. In the interlacing, input image signal to the TV receiver is subjected to field interpolation to produce an interlaced image of a double field frequency to suppress flickering of the entire screen. Also, in the interfacing, the interpolated fields are subjected to motion compensation to prevent letters from appearing skewered, a motion from appearing discontinuous, etc.
For the above motion compensation, a difference between a current frame of the input image signal and a reference frame derived from one-frame delay of the current frame is calculated and a motion vector is detected on the basis of the calculated difference. Then, in a motion-correction field positioned intermediate between the current and reference frames, pixels are shifted on the basis of the detected motion vector. Note that for the detection of a motion vector, the block matching technique is widely used as a prevailing technique in this field of art.
With the block matching technique, a current frame 80 is broken into a plurality of reference blocks 101, and a block having a closest correlation with the reference blocks in the current frame 80 is detected from a search block 103 which moves within a search range 104 of a reference frame 90, as shown in FIG. 1. Then, a deviation in position between the detected search block 103 and reference block 101 (direction and magnitude of the motion) is taken as a motion vector.
For the determination of the above correlation, a difference is calculated between each of pixels in the search block 103 and corresponding ones in the reference block 101 to determine an assessed value given by the difference, say, a sum of absolute values of differences (will be referred to as “difference absolute-value sum” hereunder wherever appropriate), for example. Next, the above operations are repeated for all the search blocks 103 to determine a smallest one of the determined assessed-value sums, namely, difference absolute-value sums. The search block 103 from which the smallest difference absolute-value sum is derived is taken as a block having the closest correlation with the reference block 101, and a vector identifiable between a pixel 113 at the origin of such a block and a one at the origin of the reference block 101 is taken as motion vector.
Next, a magnitude and direction of shifting the pixel are determined on the basis of the quantity and direction of the motion vector determined as above. Then, in a motion correction field 100 being intermediate between the current and reference frames 80 and 90, the pixel is shifted correspondingly to a shift magnitude determined based on the position of the pixel at the origin of the reference block (or the pixel is shifted in the determined direction by the magnitude of shifting of the pixel 113 from the origin). It is assumed here that the shift magnitude is a half of the quantity of the vector motion. In the motion correction field 100, the pixel will be shifted to an intermediate position between the pixel position in the current frame 80 and that in the reference frame 90. Namely, since such a motion correction field can be inserted that will smoothen the motion of the pixel continuous from the current frame 80 to the reference frame 90, so it is possible to produce an interlaced image free from display quality degradation.
It is important to note that the magnitude of a motion vector detectable with the block matching technique, that is, the range of pixel position from which a difference absolute-value sum can be derived, depends upon the sizes of the reference block 101 and search range 104. Therefore, for appropriate compensation of all image motions, the search range 104 should desirably be as large in size as possible, but the size is normally limited to a predetermined one because of the circuit scale and calculation time which should be as small and short as possible.
With the aforementioned conventional block matching technique, however, even when a real motion vector 111 to be determined possibly exists outside the search range 104, an irregular motion vector 112 different from its original motion will be determined since top priority is given to the sum of differences between the blocks 101 and 103 within the search range 104 as shown in FIG. 2. If a shift magnitude is determined directly based on the irregular motion vector 112, there will be caused an image quality degradation and failure in processing of each area.